For example, many are trying to take advantage of short sales, buying from sellers who owe more on their mortgage than the home is worth. In these deals, the lender has to agree to forgive the remaining debt, and that can take time -- at least two to three months from the time an offer is made until lender approval. And that assumes a relatively clean deal, in which there's no second mortgage. If there are any complications, expect six months or more.

"The lenders have improved but they're still terrible," said Richard Smith, CEO of Realogy, the parent company of several franchise real estate brokers. "We started telling buyers back in January that short sales may not close in time for the credit."

Another factor slowing down closings is what gets found during inspections. The average foreclosed property comes with more maintenance and repair problems than conventional sales. Fixing those can take time, said Rick Davidson, CEO of Century 21.

Meanwhile, all deals these days are at risk because of appraisal issues. With home prices still shaky, mortgage lenders have gotten strict about making sure that the sale price is not inflated.

"New mortgage money is so concerned about pricing, they require another appraisal a couple weeks before closing," said Smith. "They're consumed with value." Lining up an appraiser and getting a report can add days, even weeks to the time it takes to close.

Congress is mulling an extension that would allow people to finish their transactions as late as Sept. 30 and still claim the credit -- as long as they signed sales contracts before the original April 30 deadline.

"Brokers and agents are concerned that if Congress does not pass an extension, many of these homes will end up back on the market, defeating the whole purpose and benefit of the tax credit," said Tara-Nicholle Nelson, a spokeswoman for real estate website Trulia.

But the extension is hardly a sure thing, according to Regan Lachapelle, deputy communications director for Senate Majority Leader Harry Reid, who introduced the measure. "We are still working to get the votes we need," said Lachapelle.